chore: add community health files

Add MIT LICENSE (root, backend, frontend), CONTRIBUTING.md, CODE_OF_CONDUCT.md,
SECURITY.md, GitHub issue templates (bug/feature), and PR template.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,21 @@
+## Summary
+
+<!-- What does this PR do? One or two sentences. -->
+
+## Type of change
+
+- [ ] Bug fix
+- [ ] New feature
+- [ ] Refactor / cleanup
+- [ ] Docs
+- [ ] Chore / tooling
+
+## Testing
+
+<!-- How did you verify this works? -->
+
+## Checklist
+
+- [ ] I've tested the affected path manually
+- [ ] No `.env` secrets are committed
+- [ ] Relevant docs updated (see `../temetro/docs` if needed)